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Leveraging WiMAX into LTE Success
By Todd Mersch, Director of Product Line ManagementIn business and technology, inflection points are a rare occurrence. Furthermore, the identification of inflectionpoints while they are still in progress is even rarer. The mobile broadband industry is at one of theseexceptional inflection points right now, but my chief concern is that parties on both sides of the competitivefence will get stubborn and miss the opportunity in front of them.
The inflection point I reference is the bifurcation of markets that thirst for the next generation of mobilebroadband serviced both by WiMAX and Long-Term Evolution (LTE) technology.
For those involved in either WiMAX or LTE, the recent past has more often than not been a focused debateabout which of these Orthogonal Frequency Division Multiplexing (OFDM)-based radio technologies is betterthan the other. I am not here to espouse the technical superiority of one or the other. The fact is, both WiMAXand LTE are finding their place in the market and both will enjoy deployments.
However, it is an undeniable fact that there is unprecedented global alignment from mobile operatorsthroughout the world behind the LTE standard. In fact, there are dozens of announced LTE migration plans. Assuch, WiMAX-only solution providers stand to miss out on this massive market opportunity if they do notevolve their WiMAX solutions to LTE and soon.
The good news is that for a WiMAX radio access network (RAN) equipment provider, the creation of an LTEsolution is -- pardon the pun -- an evolution of their WiMAX portfolio, not a complete do-over. However, time isof the essence since the very public plans of Verizon in the United States and NTT DoCoMo in Japan arespurring a flurry of activity by competing operators who do not want to be left behind. The opportunity is therefor WiMAX companies to leverage their real-world expertise in deploying OFDM networks, their proven OFDMintellectual property, and their existing economies of scale from on-going WiMAX design wins in order todifferentiate themselves from the traditional Tier 1 Network Equipment Providers (NEPs) who often dominate
the GSM landscape.
So what does it take to evolve a WiMAX base station to LTE? There are two key areas of focus. The first is theradio and physical layer (PHY) subsystem and the second is the upper-layer protocols and security aspects.The remainder of this article examines the necessary modifications and areas of re-use for both of thesesubsystems.
Radio and PHY Subsystems
The radio and PHY subsystems from WiMAX and LTE are, simply put, very similar. They share a commonunderlying technology, OFDM, and the migration of the existing WiMAX radio and PHY to an LTE version isnot as significant a transition as one might imagine. Key similarities between a WiMAX and LTE radio andphysical layer are as follows:
1. OFDMA Downlink: both WiMAX and LTE utilize the same transmission techniques in the downlinkdirection, including support for 64QAM modulation
2. MIMO Support: both WiMAX and LTE utilize multiple input multiple output (MIMO) radio technology toincrease bandwidth and reduce signal-to-noise ratio
3. Overlap in channel and frequency ranges: both support 5MHz and 10MHz channel bandwidths andtransmission in frequency bands in the 2GHz range
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However, despite these commonalities there are modifications required to transition from a WiMAX radio andPHY subsystem to an LTE one. Interestingly, in a lot of cases these enhancements align with the move fromMobile WiMAX 1.0 standards to the new Mobile WiMAX 1.5 capabilities, so a move to LTE now aligns withsupporting WiMAX 1.5 in the future as well. Enhancements required to migrate to LTE in the radio and PHYsubsystem are as follows:
1. Replacing OFDMA with SC-FDMA in the Uplink (UL): the 3rd
Generation Partnership Program(3GPP) has chosen to use Single Carrier Frequency Division Multiple Access (SC-FDMA) in the ULin LTE. SC-FDMA has a significantly lower peak-to-average power ratio (PAPR) and allows for arelatively high degree of commonality with the downlink OFDM scheme including reuse of the sameradio parameters.
2. Support for FDD in addition to TDD: the latest WiMAX standards allow for FDD although themajority of existing equipment and approved interoperability profiles are only in TDD.
3. Support for 4x4 MIMO: representinganother enhancement that aligns with the migration to WiMAX1.5, current WiMAX solutions will need to add support for 4x4 MIMO to fully comply with LTEspecifications. This, however, is an upgrade that may be put on the roadmap since most initialsolutions and deployments will be 2x2 MIMO.
4. Increased flexibility in channel bandwidths and frequencies: LTE from the start is flexible enoughto support channel bandwidths from 1.25MHz up to 20MHz and for transmission in frequencies from700MHz up.
5. Smaller frame size: with an eye on support for voice services, the LTE frame size is 1ms versus a5ms frame size in WiMAX. The smaller frame size leads to a challenging timing requirement and theneed for real-time performance up into the Medium Access Control (MAC) layer.
6. Increased mobility: the LTE standards require support for mobility up to 500kmph, which is not arequirement for WiMAX systems today.
Upper Layer Protocols
Despite the longer list of differences, the enhancement from WiMAX radio and PHY subsystems to LTEequivalents is an incremental upgrade. The changes required are generally extensions of existing capabilitiesin the WiMAX radio and PHY, and this is an area of deep expertise in WiMAX equipment providersorganizations. However, as one moves up the stack into the higher layer protocols and the application, the
knowledge gap becomes larger and the sheer time required to transition becomes greater.
Figure 1 places the WiMAX RAN architecture side-by-side with the LTE equivalent.
Figure 1: WiMAX and LTE RAN Architectures Compared
WiMAX RAN
eNodeBeNodeB
Serving GW
S1-MME S1-U
MME
S1-U
X2
S1-MME
LTE-Uu LTE-Uu
ASN GW
R6 R6
R8
R1 R1
LTE RAN
BSBS
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The similarities include a flat end-to-end all-IP access network, radio resource management resident in thebase stations, and support for direct communication among base stations to coordinate handover and radioaspects. However, when one looks a little deeper, the differences begin to emerge. Figure 2 places thesoftware architectures of WiMAX and LTE base stations side-by-side.
Figure 2: WiMAX and LTE Base Station Software Compared
As is highlighted in Figure 2, the software architectures begin to diverge significantly at the Layer 2 and Layer3 protocols, both in the control and data planes. Critical differences are summarized in Table 2.
Interface WiMAXProtocols
LTE Protocols LTE Key Differences
BS to UE 16CNTL, MAC,PHY
RRC, PDCP, RLC,MAC, PHY
L2 more complex with segmentation /desegmentation at RLC layer
MAC and MAC scheduler operate at 1msframe size
Security requires support for Snow3G at
PDCP layer RRC and MAC must support FDD andTDD profiles
Specific MAC channels for differentservice types including multicast /broadcast services
Completely different encode / decodeschemas and protocol informationelements
BS to BS R8, GRE, UDP,IPSec, IP
X2AP, SCTP, eGTP-u,IPSec, IP
Reliable transport over SCTP significantlymore complex than UDP
eGTP-u for tunneling the data fromeNodeB to eNodeB in LTE versus GRE inWiMAX
Minimal control plane latency (X2) tosupport higher speed mobility services
RRM related messages carried over X2versus relayed in the ASN GW in WiMAX
Completely different encode / decodeschemas and protocol informationelements
PHY
MAC
StackManager
IP
X2AP
SCTP
RRC
RLC
S1APeGTP
Dynamic
Resource
Allocation
Radio
Admission
Control
Connection
Mobility
Cont.
RB
ControlRRM
eNB Measurement
Config &
Provision
eNodeB Application
PDCP
IPSec
PHY
MAC
StackManager
IP
X2AP
SCTP
RRC
RLC
S1APeGTP
Dynamic
Resource
Allocation
Radio
Admission
Control
Connection
Mobility
Cont.
RB
ControlRRM
eNB Measurement
Config &
Provision
eNodeB Application
PDCP
IPSec
PHY
MAC
StackManager
IP
R8
UDP
16 CNTL ASN CNTL GRE
Dynamic
ResourceAllocation
Radio
AdmissionControl
Connection
MobilityCont.
RBControl
RRMMeasurement
Config &Provision
WiMAX BS Application
IPSec
IPCS
PHY
MAC
StackManager
IP
R8
UDP
16 CNTL ASN CNTL GRE
Dynamic
ResourceAllocation
Radio
AdmissionControl
Connection
MobilityCont.
RBControl
RRMMeasurement
Config &Provision
WiMAX BS Application
IPSec
IPCS
WiMAX BS LTE BS
Layer 1 Layer 2 / Layer 3 Application
Key
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Interface WiMAXProtocols
LTE Protocols LTE Key Differences
BS to CoreNetwork
ASN CNTL, GRE,UDP, IPSec, IP
S1AP, SCTP, eGTP-u,IPSec, IP
Separate control and data plane paths Reliable transport over SCTP significantly
more complex than UDP eGTP-u in the data path versus GRE Completely different encode / decode
schemas and protocol informationelements
From Table 2, it is clear that the gap widens as one looks further into the details of the upper layer functionalityin an LTE eNodeB versus a WiMAX BS. The best area of re-use for a WiMAX solution provider will comewithin the MAC, and more importantly the MAC scheduler. Since both WiMAX and LTE utilize OFDMA in thedownlink, the MAC schedulers for downlink transmission may be leveraged from WiMAX to LTE.
Beyond the schedulers, however, LTE requires essentially a whole new set of protocols even into the transport(i.e. SCTP vs. UDP) and the time to develop these from scratch would force the WiMAX base station providerto miss the LTE window of opportunity to leverage their differentiating experience, tool sets (e.g., element
management systems, performance management systems, etc.), and existing economies of scale. All is notlost, however, as there exists a burgeoning ecosystem of solution providers that support the entire suite of LTEprotocol stacks, helping speed time-to-market for LTE network equipment.
Conclusion
The time for WiMAX solution providers is now, for the LTE market is expanding quickly. Deployments arestarting and trials are broadly underway. It would be pennywise and pound foolish to waste energy trying toargue the superiority of one technology solution over the other when there is such a phenomenal opportunityfor companies with key WiMAX assets to expand their addressable market to the over 4 billion GSMsubscribers worldwide. WiMAX solution providers have the OFDM experience and intellectual property tocompete and win large chunks of LTE infrastructure rollouts. By leveraging their existing radio and PHYsystems and working with the ecosystem of upper layer protocol software providers, a WiMAX equipment
vendor can rapidly get to market and compete for the next wave of LTE design wins.
About Continuous Computing
Continuous Computing
is the global source of integrated platform solutions that enable network equipmentproviders to overcome the mobile broadband capacity challenge quickly and cost effectively. Leveraging morethan 20 years of telecom innovation, the company empowers customers to increase return on investment byfocusing internal resources on differentiation for 3G, Long Term Evolution (LTE), Femtocell and Deep PacketInspection (DPI) applications. Expertise and responsiveness set the company apart: only ContinuousComputing combines best-in-class ATCA platforms with world-famous Trillium
protocol software to create
highly-optimized, field-proven wireless and packet processing network infrastructure. www.ccpu.com
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